1. Field of the Invention
This invention relates to processes for manufacturing methallyl chloride.
2. Discussion of the Background
Methallyl chloride (3-chloro-2-methyl-1-propene) is manufactured industrially by reacting isobutene with chlorine in the gas phase. The reaction is carried out in a cooled tubular reactor at temperatures of c. 100.degree. C. (in particular, under 100.degree. C. to the extent possible) and at approximately atmospheric pressure. The required reaction time is in the range of 0.5 sec. to several seconds. To avoid further chlorinating the methallyl chloride, a slight excess of isobutene is maintained at all times. The starting materials are introduced to the reactor through a nozzle having two outlets.
Attempts to atomize liquid isobutene as an indirect cooling means have no been successful. This technique leads to increased formation of undesirable higher-chlorinated products, evidently a result of poor intermixing of the reactants (1975 "Ullmanns Encyklopaedie der technischen Chemie", 4th Ed, pub. Verlag Chemie, Weinheim, Vol. 9, pp. 472 ff.).
Attempts by A. Striegler to prepare methallyl chloride on a laboratory scale from isobutene and chlorine have revealed that irregularities occur in the reaction which are marked by a sudden unexplained temperature rise and result in a marked increase in undesired higher-chlorinated products (1957, "Erfabrungen beider chlorierung von Isobutylen", in Chem. Techn., 9, 523 ff.). Such irregularities have been also observed, with varying frequency and duration, in industrial-scale operations.
These uncontrolled effects can be suppressed by adding certain amounts of oxygen to the reaction mixture. The procedure is set forth in detail in Ger. Pat. No. 3,402,446. Although the amount of oxygen is small according to this method (0.001-1 vol. % based on the volume of the gaseous starting materials), the feed of oxygen can lead to problems, particularly if the oxygen is supplied as air. In particular, the added oxygen exits the reaction system practically unreacted. It is thus present as a "residual gas stream" in the product refinement after the chlorine is washed out and the remaining reaction product is condensed out.
Depending on the refining techniques used, this "residual gas stream" contains chlorinated hydrocarbons and/or unreacted isobutene. Chlorinated hydrocarbons necessitate meeting generally stringent environmental regulations (in the FRG see, e.g., the "Erste Allgemeine Verwaltungsvorschrift zum Bundesimmissionsschutzgesetz", entitled "Technische Anleitung zur Reinhaltung der Luft (TA Luft)" (Edition of 27 Feb. 1986). Unreacted isobutene presents pollution problems and also the problem of keeping the isobutene-oxygen (or isobutene-air) system outside its explosive limits. The presence of chlorinated hydrocarbons and/or unreacted isobutene entails increased costs of manufacturing the methallyl chloride.
There is thus a strongly felt need for a process for producing methallyl chloride which does not suffer the disadvantages of existing processes.